A conventional periodical driving system 10 is shown in FIG. 1 and generally includes a wheel 11 which includes a plurality of radial driving slots 111 and recesses 112 defined alternatively along a periphery of the wheel 11, and a driving disk 12 includes a curved member 121 and a pin 122. When the wheel 11 is a stationary status, the curved member 121 is engaged with one of the recesses 112 and the pin 122 is located in an opening one of the driving slots 111. However, when the pin 122 enters into the driving slot 111, only half of the curved member 121 is engaged with the recess 112. In other words, there is a part of the recess 112 is not engaged with the curved member 121. The wheel 11 is easily rotated due to gravity or foreign impact because the driving disk 12 does not effectively hold the wheel 11 at the stationary status. Once the wheel 11 is unintentionally rotated, the related parts connected with the wheel 11 move and may cause severe impact between parts or even injury to the users. Besides, the single pin 122 can only be engaged with the driving slot 111 once per revolution of the wheel 11, and this consumes too much time. As shown in FIG. 2, another conventional periodical driving system 20 includes a wheel 21 with a plurality of radial driving slots 211 and the driving disk 22 has two pins 221 which are alternatively engaged with the driving slots 211. Therefore, the wheel 21 can be held every half revolution. Nevertheless, in order to allow the pins 221 to enter into the driving slots 211 easily, each driving slot 211 has a divergent and curved opening 212 which is adverse for the pins 221 to hold the wheel 21 at its stationary status. The pins 221 may slip off from the openings 212 due to gravity of the wheel 21 or foreign impact to the wheel 21.
The present invention intends to provide a control device for periodical driving system and the control device includes two driving protrusions which can be effectively stayed in the recesses of the wheel of the system to hold the wheel at its stationary status.